The use of thin plastics films to form large areas of transparent glazing is becoming an important substitute for glass. This is not only from the cost and weight point of view, but also due to the increased thermal and optical performance that can be achieved with double, triple, and even multiple glazing using this material. Hence thermally efficient structures can now be frabricated at a fraction of the cost and weight of equivalent systems, which use traditional material such as glass, fibre glass or rigid plastics.
A typical panel consists of a framework, usually metal or plastics extrusion, over which the thin film, to form the glazing, is secured and tensioned. Problems have been experienced with this form of glazing mainly due to a short life span brought about by degradation of the plastics material under ultra-violet effects of the sun's radiation, and the lack of an adequate means of securing the film material to the framework.
These difficulties have been overcome to some extent by the introduction of more durable plastics and the use of special clip for tensioning the thin film to the framework, in a dynamic manner, such a clip being the subject of U.K. Pat. No. 1,586,247.
The more durable plastics film which has been introduced in quite capable of withstanding fairly high temperatures, up to 177.degree. C., for short periods of time with a maximum working temperature of perhaps 80.degree. C. depending on film type. It is also capable of lasting for long periods of time, and in excess of fifteen years even when exposed to intense ultra-violet radiation. Under these conditions the film will show a high enough percentage of elongation to break after ten years' exposure to allow a few more years of useful service (the term `elongation of break` is presently one of the recognised methods by those knowledgeable in the field of plastics film technology for testing the tensile strength of a plastics film. It is a measure of the percentage elongation that can be applied to the film before it fails). As a plastics film ages under outdoor weathering, the percentage of elongation of break, which may be of order of 200% for an exposed piece of film, will drop to perhaps 50% after fifteen years, at which time its impact resistance is virtually gone and it will fail with the slightest application of load, i.e. snow, hail, wind or otherwise.
It has been found, however, that the new generation of plastics films still encounter ageing problems, and it has been shown that the ageing process is most likely to occur in the region where the film exits the chip fixing retaining the thin film at the perimeter of the glazing frame, for example the clip of U.K. 1,586,247 above referred to.
In particular premature failure has been found to occur where the film passes over those surface portions of the clip fixing which are exposed to direct radiation and thus under the influence of high temperatures and ultra-violet radiation.
The result is that such surface portions heat up as they absorb both the infra-red radiation of the sun's spectrum, and also the visible radiation which is converted into thermal energy when it impinges on this part of the clip fixing surface.
On a bright sunny day it is possible for the temperature of the exposed clip fixing surface portions to become substantial. The film passing over the surface is itself therefore elevated to the same temperature, and it is this high temperature coupled with the exposure to ultra-violet radiation that causes the film touching the surface at that point to age extremely quickly, the failures have been recorded within three years on film that would otherwise last for many years.
The result of this simultaneous exposure to U.V. and high temperature being an ageing process which causes the film to become brittle and eventually fail. When tested the value of elongated to break for the film in the region of failure will be below the percentage mark necessary for continuing useful life. Film in very close proximity to this area, however, will still be quite serviceable and when tested exhibit a high value of elongation to break depending on the number of years exposure.